We recently reported that normal human serum contains IgA antibody with specificity for enterochelin, the iron transport compound produced by Escherichia coli and Salmonella typhimurium. Bacterial cells with defective or relatively inefficient enterochelin systems are unable to grow in complement-inactivated serum because of this antibody. The antibody can be adsorbed out of whole serum with UV-irradiated E. coli cells, provided the cells had been able to synthesize enterochelin and possessed at least one of the major porin proteins, the products of the ompC and ompF genes, in their outer membrane. Affinity chromatography has been employed to partially purify the antibody. The antibody will be further characterized; experiments to test its (1) specificity, including its activity against unrelated iron transport compounds produced by other microbes, (2) ability to associate with several cell strains, each defective in a different outer membrane component, (3) ubiquity in mammalian sera, (4) ability to confer protection by passive immunization, and (5) its possible role in bactericidal reactions in complement active serum will be tested. The role of the porin proteins in bacteriostasis will be examined to further understand the antibody and to provide basic information regarding bacterial iron assimilation via the enterochelin system. The possible involvement of porin protein in enterochelin movement to the cell exterior and possible subsequent release from the cell and/or ferrienterochelin uptake will be examined.